The changes are in line with the Urban Redevelopment Authority (URA) cap of six tenants for rented-out private residential properties, HDB said.

Families - even those with more than six members - can continue to rent whole HDB flats. But those who rent bedrooms must follow the new cap.

The new ruling will apply for all new and renewal applications submitted from May 1.

For pre-existing rentals, the revised cap will apply upon renewal or when there are changes in the occupants.

The current rule requiring flat owners to get HDB's approval before renting out their entire unit will also apply to owners of commercial properties from May 1, to ensure eligibility conditions are met.

Replying to TNP's queries, an HDB spokesman said: "The revision is part of HDB's efforts to ensure there is no overcrowding and that the living environment in our housing estates continues to be conducive for our residents.

"From 2015 to 2017, HDB received 2,290 cases of feedback regarding the renting out of flats or bedrooms.

Mr Tay said of the new ruling: "It will be a welcome change to some residents, especially those who live next to flats with up to nine occupants."

A resident who lives in such a flat in a Yishun Street 22 block and who wanted to be known only as Madam Lee, 52, said: "Of course, it can get noisy sometimes, especially if the tenants don't cooperate with each other in being considerate to their neighbours."

Property agent Ku Swee Yong said the new ruling could benefit property agents.

He said: "In the past, a small business with maybe nine employees could squeeze all nine into one unit. With the new ruling, they will be required to get two units, which means more business for us."

He felt the ruling has come at a good time to help boost the weak rental market with an influx of available flats to cater to the new demand.

Professor Yu Shi-Ming, who heads the Department of Real Estate in the National University of Singapore's School of Design and Environment, was not surprised by the HDB move as it had followed a similar ruling on private property last year.

He said: "It will improve conditions. Imagine having flats of nine tenants on your floor with the implications on the demand for lifts and the amount of trash generated.

"This is a welcome change, but policy is one thing. The implementation of the ruling remains to be seen."